The photoproducts and hydrolysis products of butachlor in water were identified by gas chromatography/mass spectrometry. When exposed to UV light, butachlor in aqueous solution was rapidly degraded, giving at least 11 photoproducts as a result of dechlorination with subsequent hydroxylation or cyclization processes. The chemical structures of nine degradation compounds were identified on the basis of mass spectrum interpretation and literature data. Major photoproducts are identified as 8-ethyl-1-butoxymethyl-4-methyl-2-oxo-1,2,3,4-tetrahydro-quinoline, 2-hydroxy-2',6'-diethyl-N-(butoxymethyl) acetanilide, and a compound related to butachlor. Minor photoproducts are identified as 2,6-diethylaniline; 1-acetyl-7-ethylindole; N-(2,6-diethylphenyl)-N-(butoxymethyl)acetamide; 2-oxo-N-(2,6-diethyl-phenyl)-N-(butoxymethyl)acetamide; 1-hydroxyacetyl-2-butoxyl-3-methyl-7-ethylindole; 1-acetyl-2-butoxyl-3-methyl-7-ethylindole; and two compounds with the chemical structure unknown. The half-lives of butachlor UV photolysis were 7.54, 10.56, and 12.22 min in deionized water, river water, and paddy water, respectively. The half-lives of butachlor hydrolysis at pH 4, 7, and 10 were 630, 1155, and 1155 days at 25 +/- 1 degrees C, respectively. A hydrolysis product at pH 4 was identified by GC/MS to be 2-hydroxy-2',6'-diethyl-N-(butoxymethyl) acetanilide.